With the gradual exhaustion of resources on the earth and the incensement of serious environmental problems, more and more attentions are drawn to the utilization of renewable energies around the world. Microgrid has emerged as an energy integration to improve the penetration of renewable energy resources and distributed energy resources in an energy supply system. Each microgrid consists of different types of distributed energy resources (DGs) which comprise micro-turbines, wind generators, photovoltaics, fuel cells, energy storages as well as load and protection devices.
Microgrids are integrated to a synchronization AC grid via power-electronic converters, typically inverters; they exchange power with the main grid via the point of common coupling, improving the reliability of power supplies. Since the microgrid is a distributed system of small scale, and closes to loads, which can enhance the system security, reduce the grid losses and greatly increases the energy utilization efficiency, it is a promising generation mode satisfying the development of smart grid in the future. Under normal conditions, the microgrid is connected to the main grid which provides voltage and frequency references. The most attractive advantage of microgrid is to island itself from the main grid irrespective of planned or spontaneous events. The peer-to-peer control mode adopting a droop control does not require a dominant distributed generation and communication links, and thus draws wide attention. When the microgrid is required to transfer from a grid-connected mode to an islanded mode, the distributed generations can automatically share the active power and reactive power in the microgrid. However, the droop control is a proportional discriminatory control, and will cause voltage deviation from the reference value, which results to secondary voltage control. At present, voltage restoration is implemented in the centralized control scheme and the multi-agent-based distributed control. The centralized control scheme is based on a central controller, requires a complex communication network, and suffers from huge data-handling. Furthermore, the single-point failure and distributed feature of the renewable energy may both result in the change of the communication topology, which further challenge the reliability of the central controller. On the contrary, the multi-agent-based distributed control scheme only exchanges information with its immediate neighbors, realizes efficient information sharing, and finally achieves quicker decision making and operation with a simple communication structure.